GEAIN-DUST EXPLOSIOlSrS. . 47 



is similar to those used in experiments in the special mill built at The Pennsylvania 

 State (.'allege for the piupose of this investigation. 



This company reports that no damage has been done, and, so far as they know 

 explosions have not been transmitted beyond the safety pipe and damper. 



The double damper as installed in the present series of tests did 

 not give the desired effect, probably because the two sets of revolv- 

 ing paddles were too close together. If the flame can pass one set 

 of blades, it is likely that it can pass an adjacent set of blades 

 before it has been extinguished. It seems reasonable to suppose, 

 however, that a sufficient volume or extent of passage between the 

 two revolving dampers may cause the flame to expend its energy to 

 such an extent that the second damper would be able to check it 

 absolutely. 



The following conclusions can be drawn: 



1. A single revolving damper is a fairly effective device for pre- 

 venting the violent propagation of flames, but is by no means an 

 absolute preventive. 



2. A double revolving damper does not appear to serve the pur- 

 pose of a fire trap any better than a single damper. 



RELIEF VALVE AND PIPE. 



Although the three tests with the rehef valve and pipe leading 

 from the hopper of the attrition miU to the outside of the building 

 showed that this device was not effective in preventmg the propa- 

 gation of flames, even past the double revolving damper, it is quite 

 possible that a larger size of valve and pipe may aid materially in 

 accompMshing the desired result. The principle of permitting the 

 immediate escape of confined gases that have just been ignited 

 seems to be a proper one on which to base a preventive device. 



Conclusions : 



1. The rehef valve and pipe as installed alone are not successful 

 in preventing flame propagation. 



2. In view of the correct principle involved, it may be worth while 

 to try changes in the design and location of the rehef valve and pipe. 



A study of the foregoing discussion of the tests performed at the 

 experimental plant of the college may lead to the objection that not 

 enough attention was paid to the quantitative side of the problem, 

 involving such factors as the exact amount of grain fed into the 

 mill, the rate of feeding, the physical and chemical analyses of the 

 material, the minimum temperature and minimum amount of heat 

 to produce ignition, the velocity and analysis of the dust-laden gases 

 at various points between the attrition mill and the end of the con- 

 veyor, the pressure developed by the explosion, the minimum 

 humidity and the minimum amotuit of inert gases to prevent igni- 

 tion of inflammable dusts, and the hmits of inflammability for vari- 

 ous dusts. To such a criticism the authors would reply that the 



